diff --git a/src/calibre/db/tests/writing.py b/src/calibre/db/tests/writing.py
index 3f76249d91..227ce721d0 100644
--- a/src/calibre/db/tests/writing.py
+++ b/src/calibre/db/tests/writing.py
@@ -426,10 +426,7 @@ class WritingTest(BaseTest):
         self.compare_metadata(nmi, oldmi, exclude={'last_modified', 'format_metadata', 'formats'})
         cache.set_metadata(1, mi2, force_changes=True)
         nmi2 = cache.get_metadata(1, get_cover=True, cover_as_data=True)
-        # The new code does not allow setting of #series_index to None, instead
-        # it is reset to 1.0
-        ae(nmi2.get_extra('#series'), 1.0)
-        self.compare_metadata(nmi2, oldmi2, exclude={'last_modified', 'format_metadata', '#series_index', 'formats'})
+        self.compare_metadata(nmi2, oldmi2, exclude={'last_modified', 'format_metadata', 'formats'})
 
         cache = self.init_cache(self.cloned_library)
         mi = cache.get_metadata(1)
diff --git a/src/calibre/db/write.py b/src/calibre/db/write.py
index 222e88dafa..dfe15e108c 100644
--- a/src/calibre/db/write.py
+++ b/src/calibre/db/write.py
@@ -233,13 +233,19 @@ def one_one_in_other(book_id_val_map, db, field, *args):
 def custom_series_index(book_id_val_map, db, field, *args):
     series_field = field.series_field
     sequence = []
-    for book_id, sidx in iteritems(book_id_val_map):
+    for book_id, sidx in book_id_val_map.items():
         if sidx is None:
             sidx = 1.0
         ids = series_field.ids_for_book(book_id)
         if ids:
             sequence.append((sidx, book_id, ids[0]))
-        field.table.book_col_map[book_id] = sidx
+            field.table.book_col_map[book_id] = sidx
+        else:
+            # the series has been deleted from the book, which means no row for
+            # it exists in the series table. The series_index value should be
+            # removed from the in-memory table as well, to ensure this book
+            # sorts the same as other books with no series.
+            field.table.remove_books((book_id,), db)
     if sequence:
         db.executemany('UPDATE %s SET %s=? WHERE book=? AND value=?'%(
                 field.metadata['table'], field.metadata['column']), sequence)